A typical example of the cogeneration systems of the type concerned is disclosed in, for example, JP 08-004586 A. The disclosed cogeneration system includes a gas engine, a generator driven by the gas engine to generate electric energy, and a hot-water heat exchanger (waste-heat heat exchanger) for transmitting waste heat from the gas engine to cold water to thereby produce hot water.
Basic operation control systems for such cogeneration systems includes a so-called “heat-demand priority type” in which operation pattern of the cogeneration system is set according to an amount of hot-water supply required at the output side of the cogeneration system. In this operation control system, when the demand for hot-water supply is not present or low, the gas engine is stopped so as to prevent the temperature of the gas engine from increasing above a predetermine value. Thus, the heat-demand priority type operation control system gives priority to hot-water supply over electricity supply. However, with the gas engine thus stopped, the cogeneration system cannot supply electric energy when required, for example, in case of electric power failure.
It is, therefore, an object of the present invention to provide a cogeneration system which is capable of continuing operation of an engine even when the demand for hot-water supply is not present.